Method of applying a binder to electro phoretically deposited porous matrix



Nov. 18, 1969 E. J. AIRoLA 3,479,268

` METHOD OF APPLYING A BINDER TO ELECTROPHORETICALLY DEPOSITED FOROUS MATRIX Filed Aug. l. 1966 United States Patent O M 3,479,268 METHOD OF APPLYING A BINDER TO ELECTRO PHORETICALLY DEPOSITED POROUS MATRIX Ellis .lohn Airola, Lynn, Mass., assignor to General Electric Company, a corporation of New York Filed Aug. 1, 1966, Ser. No. 569,387 Int. Cl. C23f 17/00; C2311 13/00; B01k 5/02 U.S. Cl. 204-181 4 Claims ABSTRACT OF THE DISCLOSURE A uniform coating is applied and bonded to the surface of the article by first electrophoretically depositing a porous coating matrix and then dispersing through the matrix and curing a liquid non-metallic binder in the form of nely divided droplets while maintaining around the droplets substantially the same solvent balance of the binder.

This invention relates to the coatirg of an electrically conductive article and, more particularly, to a method for applying to a metallic member a corrosion resistant coating protective at elevated temperatures.

The developing metallurgical art is identifying improved iro'n base superalloys each having various combinations of strength properties and corrosion resistance. Frequently, however, some sacrifice of strength properties are required to achieve improved corrosion resistance which becomes inherent in the alloy by reason of its composition.

An approach other than alloy composition control to provide a corrosion resistant article, such as is used in relatively high temperature operating power producing apparatus as a gas turbine, is to manufacture the article from an alloy having the desired strength properties. Then a corrosion resistant coating is applied to the surface of the article for protection when exposed to corrosive atmospheres.

For large load carrying structures such as gas turbine frames and compressor rotor wheels, a thickness range for such a coating can be maintained relatively easly because the range is relatively broad. Considerations of economics and adherence usually are the controlling factors rather than corrosion resistance for very thin coatings Within narrowly controlled ranges. Consequently, metallic or non-metallic coatings such as are obtained by various spray or dip paint and cure type of operations have been reported.

For use with relatively small parts having critical contours and dimensions, such as compressor blade members for use in small gas turbine engines, coating methods must maintain minimum and maximum thicknesses within relatively narrow ranges. In addition, such methods must not create a buildup either at the leading or trailing edge of an airfoil shaped member. Such a buildup can seriously affect the airflow pattern and the efliciency of the entire gas turbine engine.

Although pack cementation type processes have been used for some of the higher temperature operating components, intermediate temperature operating components such as compressor blades cannot tolerate some of the temperatures required in pack cementation processes. Consequently, certain types of spray or dip coatings have been applied and subsequently heat cured. Unfortunately, the coatings applied by spray or dip methods tend to Iresult in higher concentrations of coating material at the leading and trailing edges.

It is a principal object of the present invention to provide an improved method for developing a uniformly thick coating over an article surface, irrespective of the complexity of the shape of the surface, the coating having 3,479,268 Patented Nov. 18, 1969 ICC inherent or oxidation resistance at temperatures up to about 1000 F.

Another object is to provide an electrophoretically deposited portion on an article as a coating base and then binding that base to the surface of the article with a corrosion resistant material.

These and other objects and advantages will be more readily understood from the following detailed drawing, description and examples which are typical of but not meant to be any limitation on the scope of the present invention.

The drawing is a partially diagrammatic view of apparatus used in the practice of the method of this invention.

It has been recognized that an improved and uniform coating can be obtained to fulfill the above objects by first electrophoretically depositing a porous coating matrix onto a cleaned surface of the article and then binding the matrix to the article by impregnating the matrix with an atomized binder deposited from dispersed nely divided droplets or aerosol. The matrix is electrophoretically deposited preferably from an organic solvent such as an alcohol which has as its electrolyte a small amount of an inorganic material.

The present invention will be particularly described in connection with an electrophoretically deposited aluminum porous matrix bonded to an iron base alloy of the A.I.S.I. type 410 stainless steel. However, it should be understood that in the practice of the present invention, any electrophoretically deposited matrix, be it metallic or non-metallic, can be applied to an electrically conductive member and subsequently bonded with an appropriate dispersed liquid binder such as in the form of a fog.

The subject of electrophoretic deposition of a wide variety of materials both of the metallic and non-metallic type have been broadly described in the patent and general literature. Typical examples are U.S. Patents 2,848,- 391, Fahnoe et al., issued Aug. 19, 1958, and 3,200,057, Burnside et al., issued Aug. 10, 1965.

Some of these and other descriptions discuss certain bonds which result solely from the electrophoretic deposition process. Generally, the electrophoretically deposited materials must be treated further such as by heating at elevated temperatures or by such heating and pressing to sinter the deposited material together as a sheet or to bond it to the surface of an article.

It has been found in the practice of the present invention that the electrophoretically deposited matrix must be in a porous condition. Under those conditions, subsequently applied binder in the form of ine, discrete droplets can be absorbed by and can impregnate the matrix. Curing of the binder will then bond the matrix to the article. The attempted co-depositionV of a binder from the same electrophoretic bath as the matrix material has not been found to be practical for purposes of this invention because the addition of such a binder drastically increases the conductivity of the bath to the point where coating is no longer practical.

It has been found that the application of a binder to the electrophoretically deposited surface must be in the form of dispersed, finely divided droplets. For the softer forms of the matrix, it is preferred that the droplets of binder be moving relatively slowly with respect to the surface of the electrophoretically deposited matrix, for example when dispersed in a fog. Fast droplet movement has been seen to disturb soft electrophoretically deposited material, producing coating faults resulting in reduction in the corrosion resistance and surface roughness of the coating. Therefore, one form of the present invention comprises the combination of a relatively soft type of electrophoretically deposited matrix to which is subse- 3 4 quently applied a finely divided binder from a fog type During deposition operation, a panel 22 was suspended atmosphere. in bath 12 Within cup 14. Cup 14 surrounding the panel Furthermore, it has been recognized that the solvent and panel 22 were connected to a source of direct elecconcentration of the container in which the fog or districal current so that the cup was anodic and the panel persed finely divided droplets are located must be conwas cathodic. The electric current was passed between trolled so that the composition of the binder is relatively panel 22 and cup 14 at various potentials and times to unchanged by evaporation of its solvent. Thus, if the deposit from the bath a matrix of a desired thickness. binder has a water base, the humidity of a coating im- For example it was found that about -l mil of matrix pregnating chamber must be controlled to maintain the is deposited for each 5 seconds at a potential of 40 v. binders water balance. and 18 milliamps current. Potential can be applied through The present invention will be more fully understood the bath up to the point where arcing will occur between from the following detailed examples which are typical the suspended particles of aluminum and the cup, thus of but are not meant as limitations on the scope of this short circuiting the system. invention. The specimens used in the examples were sheet In all of the examples listed in the following tables, panels type 410 stainless steel, 2" x .75" x .04 in size l5 the bath 12 was an alcohol bath in which spherically with a 3 square inch surface area. shaped, finely divided aluminum of 5-10 micron size was Prior to coating, the panels were degreased to remove dispersed. The inorganic salt AlCl3 Was used as the elecdirt and organic materials such as oils and greases. Then trolyte. Organic electrolytes are not included in the bath the surfaces to be coated were grit blasted with a 220 used in the method of this inventionv to inhibit coating mesh A1203 or Brite-Blast #1250 glass beads to remove breakdown during subsequent heat curing of the coating.

TABLE I Matrix electrophoretic deposition data: Solvent-Denatured Ethyl Alcohol 500 ce. (473 g.) Electrolyte-AICI; 27 mg (5x10-4M) Temperature-2835 C.

A1 Milliamps Ex. K G. Wt. percent Volts Total (avg.) /in.B Time sec. Remarks 5 1. 20 10 3.3 15 No deposit. 5 1.0 40 14 4.7 15 Coated all over. 5 1.o 40 14 4.7 10 Do. 5 1.o 60 23 7.7 10 Do. 5 1.o 40 19 5.3 5 De. 7.5 1.6 40 18 6.0 5 Do.

40 7.8 40 18 6.0 5 Coating too heavy.

1 40 mg. A1013. surface oxides. It had been found in previous tests that Itis to be noted from Table I that under the conditions electrophoretic deposition would not occur properly and 35 and using the apparatus and specimens of the examples uniformly for purposes of this invention on these panels with an ethyl alcohol bath, an aluminum concentration on a degreased but not grit blasted surface. of at least about 1 weight percent but less than 7.8 weight The apparatus used to electrophoretically deposit the percent should be used with current densities greater than matrix onto the panels is shown generally at 10 in the 3.3 milliamps per square inch when the combination of drawing. An electrophoretic bath 12 of an alcohol con- 40 aluminum concentration and voltage is low.

TABLE II Matrix Electrophoretic Deposition Data: Solvent-Methyl alcolglc. (478 g.) Electrolyte-AlCh 27 mg. (5x10-4M) Temperature- Al Milliamps Ex. G. Wt. percent Volts Total (avg.) lin.2 Time, sec. Remarks 5 1.0 5 14 4.7 15 Coated edges only.

10 2.1 10 10 3.3 15 Coated al1 but center 1o 2.1 10 12 4.0 15 Do.

10 2 1 60 15 5.0 15 Coated all over.

3.0 20 2.7 15 Coated ellbut center 15 3.0 40 14 4.7 15 Coated all over.

4.0 20 e 3.0 15 Do.

20 4.0 70 1s 6.0 5 Do.

2o 4.0 50 12 4.0 5 Do.

2o 4.o 50 5 De.

l Added 27 mg. additional A1013.

taining nely divided particles of aluminum and an in- The examples of Table II show that with methyl alcohol organic electrolyte was placed in a 4 diameter circular QS the base fur the bath, gleztef Voltages, Current deDSlstainless steel cup 14 about 4 deep along its cylindrical ues aud aluminum CODCeDFauOS are'r'equlfed tha'll Wlh side and having a funnel shaped bottom. The cup was a. ethyl alcqhol bath- T9113 for dePOSltlOu 0f muuu( P01'- connected by a hose 15 to a pump 16 which circulated t1ons useful 1n the practice of the method of th1s invention, conditions and concentrations should be adjusted to provide uniform, complete coverage.

After deposition of the uniform porous matrix such as results from practice of Examples 2-6, 13 and -15-23, inclusive, a binder was applied to selected samples. One typical and very satisfactory combination of conditions the bath through a temperature controlling heat exchanger 18 back to cup 14. The circulation was controlled by a valve means such as clamp 19 with a by-pass 21 to allow partial circulation around pump 16. For the purposes of the specimen panels, heat exchanger 18 was a water 70 jacket which controlled the electrophoretic bath temperais that Shown for Example 5 in Table 1 A Surface of ture 1u the fange 0f about Zig-35 C- The circulated and specimens prepared according to this example were treated temperature controlled bath was made to re-enter cup 14 with binder in different Waysbeneath the surface 20 of bath 12 to avoid evaporation One binder applied was an unpigmented inorganic of the alcohol solvent from the bath. binder based on a phosphate glass including chromic and phosphoric acids. 'This binder is that used in Sermetel W material commercially available. Another binder used is that more fully described in U.S.'patent application SN 509,158, filed Nov. 22, 1965, for Inorganic Glass Coating and Method for Making and assigned to the assignee of this invention. Although these binders were used in the examples, any binder suitable for the end application can be used in the practice of this invention.

In one series of tests, the binder was applied by conventional spray gun type equipment. In applications using a Fisher Sprayer -719*5 and using the finer spraying Paasche H air brush, the relatively fast moving droplets of binder disturbed the soft porous deposit of Example 6 to the point at which breaks were noted in the coating. However, when drops of binder were applied gently to the surface as can be applied from a fog atmosphere, a highly uniform, undisturbed uncured coating resulted.

After application of the binder in the proper manner, the specimens were air dried and cured at 600 F. for 1/2 hour.

Cured specimens of Example 6 were placed in a standard ASTM salt fog corrosion test chamber. After 28 days, no corrosion was noted on the coated surfaces. In fact, hydrated aluminum hydroxide was noted on the surface of some specimens showing that sacrificial action of the coating in protecting the base steel of the panel had occurred.

Thus the present invention combining the steps of uniformly electrophoretically depositing a matrix and then impregnating the matrix with finely divided droplets of binder to maintain matrix integrity provides a unique method of coating surfaces of articles such as are subject to corrosion.

Although the present invention is described in connection with specific examples, it will be understood by those skilled in the art the modifications and variations of which the invention is capable. It is intended by the appended claims to cover all such variations and modifications.

What is claimed is:

1. A method for coating a surface of an article cornprising the steps of:

(a) cleaning the surface to eliminate dirt, organic materials and surface oxides;

(b) electrophoretically depositing a porous coating matrix on the surface from a bath of suspended matrix material, the bath having as its electrolyte an inorganic material;

(c) `removing the article from the bath;

(d) drying the matrix;

(e) dispersing about the matrix a liquid non-metallic binder in the formA of finely divided droplets while maintaining around the droplets substantially the same solvent balance of the binder;

(f) holding the matrix in the dispersed droplets of binder to impregnate the matrix;

(g) removing the impregnated matrix from the dispersed droplets of binder;

(h) drying the binder impregnated in the matrix; and

then

(i) curing the binder-to bond the matrix together and to the surface of the article.

2. The method of claim 1 in which the bath is an alcohol bath, the electrolyte is A1Cl3 and the liquid binder dispersed about the matrix has an inorganic base, the binder being in the fornfof iinely divided droplets moving at a rate which will not disturb the matrix suiciently to produce coating faults upon contact between the droplets and the matrix.

3. The method of claim 2 in which the alcohol bath, in addition to the AlCl3A electrolyte, consists essentially of about 1 to less than .7.8 weight percent finely divided aluminum with the balance alcohol, the liquid binder being dispersed about the matrix in the form of a fog.

4. The method of claim 3 in which the porous coating matrix is electrophoretically deposited at a voltage of at least 20 and a current density of at least about 3 milliamps per square inch, the aluminum being in the range of at least about 3 weight percent when the voltage is in the range between 20 and 40 volts.

References Cited UNITED STATES PATENTS 2,297,691 10/1942 Carlson 117--106 2,431,629 11/1947 wind et a1. 204-181 2,848,391 8/1958 Fanno@ et a1. 204-181 2,858,256 10/1958 Fanno@ et a1. 204-181 2,872,388 2/1959 Fahne@ et al. 204-181 3,157,546 11/1964 ,Cover 117-175 FOREIGN PATENTS 5,691 2/1911 Great Britain,

JOHN H. MACK, Primary Examiner E. ZAGARELLA, IR., Assistant Examiner U.S. Cl. X.R. 117-106; 204--38 f 

